Pessimization from reduce-all-givs and move-all-movables

[Zack Weinberg]

Toon Moene <toon@moene.indiv.nluug.nl> writes:

> Jerry Quinn wrote:
>
>> Without -fmove-all-movables -freduce-all-givs, performance improves to
>> 1048 Kpos/sec, a 7% difference.
>
> This wouldn't surprise me.  Between when I introduced those flags (in
> 1997) and now we gathered quite some information on them.  It turns
> out they're mostly useful for Fortran code (subroutines) with rank-n
> dummy arguments where n>1.  Because this situation is so common in
> Fortran code, these options are enabled by default when invoking g77.

Can we come up with an algorithm that can tell whether these
optimizations will improve code or not, and enable them only where
they'll help?

zw